A power supply IC (Integrated Circuit) for supplying a power supply voltage to a load is used in many electronic devices. A synchronous rectifying buck switching regulator is an example of the power supply IC.
The switching regulator includes a switching element, a hysteresis comparator, a light-load detection circuit, and a driver circuit. The hysteresis comparator compares an output voltage of the switching regulator with a first threshold voltage and a second threshold voltage that is lower than the first threshold voltage. The light-load detection circuit determines whether a load connected to the switching regulator is in a light-load state. The driver circuit turns on/off the switching element based on a pulse signal output from the hysteresis comparator.
The hysteresis comparator shifts the second threshold voltage to a higher electrical potential by a predetermined voltage width when the light-load detection circuit determines that the load is in the light-load state. If the light-load detection circuit determines that the load is in the light-load state, the driver circuit stops turning on/off the switching element when the detection voltage reaches the first threshold voltage until the detection voltage falls to the shifted second threshold voltage. Thus, it is possible to reduce ripples of the output voltage when entering the light-load state.
In general, the power supply IC includes an output transistor. A leak current may leak out from the output transistor. For example, the leak current increases when the power supply IC is used under high-temperature circumference, which may influence an operation of a load circuit connected to the power supply IC.
FIG. 12 shows a configuration of a voltage generating circuit 200 of the related art. Referring to FIG. 12, the voltage generating circuit 200 includes an output transistor Tr and a control circuit 50.
The output transistor Tr is a PMOSFET (P-type Metal Oxide Semiconductor Field-Effect Transistor). A source and a drain of the output transistor Tr are electrically connected to a power supply node Vdd and an output node Vout, respectively. The output transistor Tr turns on/off an output of voltage from the power supply node Vdd to the output node Vout, in response to a control signal applied to its gate.
The control circuit 50 includes a comparator 51 and resistors R1 and R2. The resistors R1 and R2 are connected in series and provided between the drain of the output transistor Tr and a grounding node GND. The comparator 51 includes a non-inverting input terminal FB and an inverting input terminal COMP. The non-inverting input terminal FB receives a feedback voltage Vfb from a connection node between the resistor R1 and the resistor R2. The inverting input terminal COMP receives a predetermined reference voltage Vref. The comparator 51 amplifies a difference between the feedback voltage Vfb and the reference voltage Vref to output the control signal. The control signal is applied to the gate of the output transistor Tr.
The leak current leaks out from the drain of the output transistor Tr. A portion of the leak current. i.e., leak current I1 flows through the resistors R1 and R2 to the grounding node GND. Resistance values of the resistors R1 and R2 are set to be extremely large in order to reduce a power consumption of the voltage generating circuit 200. Thus, the voltage across the resistors R1 and R2 becomes large, even when the leak current I1 is relatively small.
The output voltage of the output node Vout is equal to the voltage across the resistors R1 and R2. Therefore, large voltage may be output from the output node Vout, when the leak current I1 increases. More specifically, when the resistance value in a series circuit of the resistors R1 and R2 is 5MΩ, even 1 μA of the leak current I1 causes the output voltage of 5V to be output from the output node Vout. Thus, malfunction of the load circuit (not shown) connected to the output node Vout may be caused. In addition, when a withstand voltage of the load circuit is lower than 5V, the load circuit may be damaged.